If you’re ever looking for a short flash game that can be played on PC (maybe when you’re bored at work/in class), can I recommend Flow? It’s simple and won’t take you ages to learn how to play. You’ll also get to see your critter grow and evolve, which is awesome. The picture below is my critter after I ate everything on every level (for the Flash game). …well… might have missed a few, actually.
And if you like the game, there’s an offline mode (around 8MB) which you can download and play at home. If you’re more into console… well, the game has been ported to PSP and PS3. It’s a nice game to kill time 😀